Airplane wings represent lifting surfaces in a flow field. Under normal operating conditions, the lift on such wings is due primarily to the large-bound vortices which appear on the low pressure side of the wing. The strength and size of the vortices depend upon their velocity, angle of attack, sweep angle, etc. The lift on the wing also depends on other characteristics of the vortex, including its trajectory, strength, and location of the vortex burst point (i.e. the point at which the vortex becomes unstable due to reaching certain critical limits of its characteristics). In lifting surfaces, it is desirable to control the lift of the surface. This will aid in, for example, the degree of maneuverability of a fighter plane. A number of devices have been proposed to control the lifting surfaces by altering the vortices to provide a vortex lift control system.
Essentially, vortex lift control systems involve stabilizing the vortex shed from the leading edge of the wing so as to direct the leading edge vorticity along the direction of the wing. Lift control is dependent upon the vortex burst point. At the location of the vortex burst, the wing lift is severely reduced. It is therefore desirable to delay vortex burst and thereby increase the wing lift.
A number of devices have been proposed to control the shedding of vortices at the leading edges of the wings. These include additional wings mounted on the aircraft fuselage ahead of the wing, and the use of tuned cavities on the leading edge of the wing. U.S. Pat. No. 4,697,769, issued to Ron F. Blackwelder et al. on Oct. 6, 1987, discloses the method and apparatus for controlling bound vortices in the vicinity of lifting surfaces. This patent discloses a piston and cylinder located in a slot of the leading edge of a delta wing in order to create a natural resonance similar to the shedding rate of the discrete vortices. Alternate embodiments disclosed in this document teach the use of audio speakers substituted for the piston, or the slot can be eliminated and a piezoelectric array embedded in the air flow to provide the disturbance. U.S. Pat. No. 4,569,494, issued to Sumio T. Sakata on Feb. 11, 1986, discloses a method for pitch control of swept wing aircraft which utilizes a fixed position vane mounted directly to the fuselage forwardly of the wings to enhance vortex formulation.
While these structures have been proposed, none have achieved a degree of active controllability that makes them practical. The permanent structures that have been proposed as passive vortex control systems adversely affect the other aspects of the aircraft's aerodynamic performance, since they are permanently deployed and may foul the air flow at different speeds.